Validate, or better immediately redirect, duplicate CCK Link field submissions

When posting a link, use case being Add Web Link on data.agaric.com, the site should check if the suggested URL is a duplicate.

This is especially useful for new modules and Drupal Planet posts that we are all likely to notice at the name time.

Kathleen will be adding a check for duplicates to the validate function, but I'm interested in ways to check for duplicate CCK field links before letting the form proceed. Not sure how to catch it really early. AJAX could check continuously for changes in the URL.

We should automatically strip off www and trailing slashes for comparison purposes.

Another option would be to offer to transform your post into a comment on the original posting of the link.

For current attempts to solve this see Prevent duplicate link submission when using CCK link module, with AJAX alerts.